Gee, this is getting confusing.

Mercenaries = Roland the headless Thompson gunner, people who engage in combat missions for the highest bidder.

Security forces = bodyguards and those pot-bellied guys who carry fake badges and unloaded guns at the mall.

I suspect we're not talking the same thing here. Civilian contractors fall in the later group or something even less military looking. Remember most military jobs are just like their civilian counterparts. Running a cafeteria or keeping the local area network up just doesn't require that much bootcamp and camoflage to do. Hence, hiring civilians.